Break AphrontDatabaseConnection dependencies on PhabricatorEnv
Summary: We pull "retries" and a doc link from PhabricatorEnv directly. Break these dependencies so the classes can move to libphutil.
Test Plan: Browsed site, triggered a schema exception and verified I still got the useful footer text.
Reviewers: btrahan, vrana
Reviewed By: vrana
CC: aran
Maniphest Tasks: T1283
Differential Revision: https://secure.phabricator.com/D3053